There have been previous approaches to charging batteries, including those mounted on a towed vehicle. Examples of such approaches are described in U.S. Pat. No. 4,647,139 (Yang); U.S. Pat. No. 4,649,332 (Bell); U.S. Pat. No. 4,829,223 (Broberg); U.S. Pat. No. 4,885,524 (Wilburn); U.S. Pat. No. 5,013,259 (Maurer); U.S. Pat. No. 5,583,414 (Lawrence); U.S. Pat. No. 5,963,013 (Watson et al.); and U.S. Pat. No. 6,166,516 (Albright). One approach to remote-battery charging includes an electrical circuit having a diode connected in series with a resistor having a resistance value within the range of 200 to 235 ohms. These components are electrically connected in series between the charging source, which is typically the positive terminal of the battery of the towing vehicle, and the positive terminal of the battery on the towed vehicle which needs to be charged. At the beginning of charging operation, the towed vehicle battery is often relatively discharged and the current draw from the towed vehicle battery is relatively significant. After a period of time, as the diode passes the current, the towing vehicle electrical system charges the towed vehicle battery, and the current draw from the towed vehicle battery is reduced. The high-wattage resistor used to reduce the current generated in the charging circuit may be large enough to result in a hazardous condition (e.g., excess heat, possibly causing a fire). Particularly, this may occur when the towing vehicle battery and the towed vehicle battery are improperly connected in reverse polarity, which creates a 24-volt potential, or when the battery on the towed vehicle becomes internally short-circuited. Although use of a high-wattage current-limiting resistor protects against such adverse conditions, a major drawback of this system is that, by including a relatively high resistance in the circuit, the normal or typical charging current is so small that the towed battery receives very little charge, even over long periods of time; this reduction in charge is directly due to the resistance and associated voltage differential across the diode/resistor combination and, to a lesser extent, the inherent resistance of the cables used to interconnect the batteries. Depending upon these resistance values, the rate of charge can be relatively insubstantial and, for relatively short towing trips, unacceptable.
Other disadvantages with the above approaches involve manufacturing, installation and ease of use in various applications. For example, in the construction business, several high-amperage batteries are often used at each construction site, so that the construction worker can rotate use of the batteries and maintain at least one of the batteries charged at all times. Where there is need to charge multiple batteries, the conventional approaches are burdensome insofar as the battery user does not have time to monitor charge time for each battery and typically does not have the time and space for buying and installing multiple battery chargers.
Accordingly, these and other design considerations have presented challenges to efforts to implement an approach to charging multiple batteries that is relatively inexpensive in terms of space and component costs, and also capable of providing an adequately high charge rate.